The Los Angeles Department of Water and Power has turned two big lakes into a monster battery capable of storing enough energy to power tens of thousands of homes. It involves using the excess wind and solar power L.A.'s renewable energy sites produce during the day to pump water from Castaic Lake uphill 7.5 miles to Pyramid Lake. Then, late in the day, when the sun goes down and the city's energy demand spikes, the water gets run downhill through hydroelectric generators at Castaic Lake.